Calcium hypochlorite (cal hypo) has been around for nearly 100 years. Initially, unrefined cal hypo, which was known as “bleaching powder”, was produced containing about 35% available chlorine. In the 1930's, manufacturers succeeded in refining cal hypo; anhydrous cal hypo was produced containing 80% available chlorine or higher. After several fires, research was done that showed that cal hypo at 70% available chlorine was less vigorous than 80% available chlorine; therefore, manufacturers limited anhydrous cal hypo to 70% available chlorine.
Anhydrous calcium hypochlorite containing 70% available chlorine was involved in many shipboard fires that occurred during the late 1960's and early 1970's and caused major property damage and loss of life. Due to these incidences, the manufacturers of cal hypo produced less reactive hydrated forms of cal hypo. See, U.S. Pat. Nos. 3,544,267; 3,895,099 and 4,053,429. They also patented hydrated salts mixed with anhydrous cal hypo to form hydrated cal hypo. See, U.S. Pat. No. 3,793,216.
Hydrated calcium hypochlorite containing at least 65% available chlorine and from about 5.5 to 16% water are available as shown in Chemical Economics Handbook, Hypochlorite Bleaches (2003) SRI International. Those containing 5.5 to 10% moisture became the most widely available commercial form and designated by the United Nations as UN 2880. Even at 10% moisture, dihydrate cal hypo is not completely saturated with water; complete saturation would be approximately 17% water. Commercial cal hypo is lower in water than saturation because higher water content would have an adverse effect on the stability of cal hypo. The loss of stability of the available chlorine in cal hypo due to the presence of moisture content, temperature and humidity is well known in the art; see, U.S. Pat. Nos. 3,544,267, 4,355,014 and 4,965,016, and Bibby and Milestone, J. Chem. Tech. Biotechnol (34A), pp. 423–430 (1984).
The commercialization of hydrated cal hypo in addition to changes in storage of the material aboard ships proposed by Clancey, Journal of Hazardous Materials, (1) pp. 83–94, (1975/76) made cal hypo safer in marine transport. This was seemingly confirmed by the work of Mandell, Fire Technology (7), pp. 157–161 (1971), that showed an endotherm for hydrated cal hypo at 40° C. according to Differential Scanning Calorimetry (DSC). This endotherm, which is not present in the anhydrous material, is thought to provide a heatsink to quench an exotherm so that no propagation occurs. This investigation also used tests incorporating lit matches, lit cigarettes, or one drop of glycerin as an ignition source for the calcium hypochlorite.
A study by Cardillo, Rev. Combust, (48) pp, 300–305 (1994), concluded that the addition of moisture lowers the temperature at which exothermic decomposition of cal hypo takes place. They also concluded that the heat generated during the decomposition would be increased. Gray and Halliburton, Fire Safety Journal (35), pp. 223–239 (2000), investigated hydrated cal hypo based on marine fires that happened in the late 1990's. They concluded that hydrated cal hypo had a lower critical ambient temperature for large quantities of material than was previously thought. Both studies confirm that hydrated cal hypo is not as stable to exothermic decomposition than was previously reported. The UN Transport of Dangerous Goods (TDG) subcommittee confirmed this using a self-accelerating decomposition test (SADT) on various manufactured cal hypos. See, UN/SCETDG/21/INF.8 (2002).
While improving the available chlorine and reducing reactivity of cal hypo has been going on for a long time, another difficulty using cal hypo, namely, the formation of calcium scale, has been equally investigated. Since the earliest use of cal hypo as a liquid bleaching compound, scale, such as calcium carbonate, has been an issue. Scale inhibitors such as sodium pyrophosphate, sodium tripolyphosphate, sodium polyacrylate, phosphonobutane tricarboxylic acid and alkali salts of PESA and/or polymaleic acid have been added to improve the clarity of cal hypo solutions or prevent the deposition of calcium sensitive soaps and detergents or reduce scaling in equipment feeding cal hypo. Mullins, in U.S. Pat. No. 5,112,521 and Faust, U.S. Pat. No. 3,669,894, also confirm what is known to those skilled in the art; it is difficult to find additives that can be admixed with cal hypo since those additives can affect stability of the cal hypo causing a loss of available chlorine especially at elevated temperatures.
Investigators have added materials that reduced dusting and increased the resistance to ignition by lit cigarettes, matches, or a drop of glycerin using spray-grained low melting hydrated inorganic salts such as: magnesium sulfate hydrates, sodium tetraborate hydrates, aluminum sulfate hydrates, and sodium phosphate hydrates; see, U.S. Pat. No. 4,146,676. Additionally, coatings of alkaline metal salts like sodium chloride and alkaline materials like calcium hydroxide may also be used to buffer compositions above pH 9. Both '676 and Pickens, WO 99/61376, agree that the presence of acidic material causes a decrease in storage stability.
Other investigators have added materials to calcium hypochlorite to provide other functions. For water treatment, Loehr, U.S. Pat. No. 4,747,978, added 0.1 to about 3% water soluble aluminum salts to increase water clarity. Pickens added hydrated aluminum salts as long as they were neutralized with an equivalent amount of hydrated borate salt. Girvan, U.S. Pat. No. 5,676,844, added hydrated borate salts or boric acid to improve the properties of cal hypo. Robson in U.S. Pat. No. 3,560,396 used spray-dried sodium nitrate preparations of cal hypo to reduce reactivity.
Only two inventive compositions containing admixed formulations with calcium hypochlorite disclose increased stability to loss of available chlorine. Jaszka, U.S. Pat. No. 3,036,013, adds a solution of a soluble salt that would react with the calcium hypochlorite to form an insoluble calcium salt at the surface of each granule. This coating would increase the stability of the particle in the presence of humidity and moisture. The coating would also slow the dissolution rate of the composition. Suitable salts include sodium silicate, sodium borate, sodium carbonate, trisodium phosphate, disodium phosphate and potassium fluoride. The composition of Murakami, in U.S. Pat. No. 4,355,014, used at least 5% calcium hydroxide to increase the stability of compositions containing at least 4% and up to 22% moisture. In both cases, the process and/or the material added result in slowing down the dissolving rate of the cal hypo. This is not a desirable effect when adding the material as a dry granular blend in a water treatment application or other applications that needs a quick dose of free available chlorine.
In response to marine fires and retail store fires that occurred during the 1990's, cal hypo manufactures and marketers continued to investigate making cal hypo safer. Girvan promoted the composition in U.S. Pat. No. 5,676,844 as less reactive based on the results of U.S. Pat. No. 3,793,216, as well as the algicidal and fungicidal benefits of the composition.
U.S. Pat. No. 6,638,446 uses the UN DOT Oxidizer test to classify their material, which contains magnesium sulfate heptahydrate as a non-oxidizer. This is also the claim by Pickens in WO 99/61376. This test, which is used to classify materials as oxidizers, has come under scrutiny due to variability based on relative humidity, cellulose source, and ignition wire composition and diameter; see, Journal of Loss Prevention in the Process Industries (14), pp. 431–434, (2001) and Journal of Safety and Environment (2), pp. 32–35, (2002). Since cellulose is hygroscopic, the hydrated water could interfere with the results of this test. The '446 patent teaches away from other past tests using a lit cigarette, a lit match, or one drop of glycerin since none of these tests is demanding due to the absence of fuel. A better test is one that is more severe and also more reproducible than the UN/DOT oxidizer test.
Reactivity in compositions shown in U.S. Pat. No. 3,793,216, using one drop of glycerin were noted by a time delay in reactions as well as the reaction itself and the amount of destruction to the sample. Glycerin as well as brake fluid are especially reactive with calcium hypochlorite. The reaction with brake fluid has been detailed chemically; see, Journal of Forensic Sciences (36), pp. 902–907, (1991). This exothermic chemical reaction initially has a delay period until a fireball erupts; the fireball lasts a few seconds and consumes all the brake fluid and much or all of the calcium hypochlorite depending on the quantity of materials used. The “freshness” of the cal hypo is also a factor in the reaction.
The rationale for changing anhydrous calcium hypochlorite to hydrated calcium hypochlorite was to make the product safer due to decomposition by a cigarette, spark, or one drop of contamination by an organic compound. The addition of water to anhydrous cal hypo changed the characteristics of the material including a unique hazardous chemical designation of UN 2880; however, hydrated cal hypo is still a highly reactive material that can be purchased by a consumer. Unsuspecting consumers are not keenly aware of the potential danger the material poses. Thus the need exists to make a calcium hypochlorite containing formulation that provides to the consumer a product that is safe to transport, store, and use. Accordingly, there is a need to improve the safety of calcium hypochlorite formulations to provide a stable, safe, fast dissolving and functional blend for water treatment and other applications.